Recently, the requirement of replacing oil-based inks or oil-based paints with aqueous inks or aqueous paints has increased, in order to improve environmental protection, safety at work, and stability. On the other hand, the requirement of replacing dyestuffs, which are coloring agents for conventional aqueous inks, with pigments, in order to improve water resistance or light fastness of a printed image is also high, and hence development and improvement of aqueous pigment dispersions as an intermediate material for producing inks for forming various images is now being performed.
As for aqueous inks for ink-jet recording, stability upon being jetted (jetting stability) and long-term storing stability etc., are more strictly required compared to other uses. That is, it is necessary for fine particles of a pigment which has as uniform a diameter as possible and is coated with a resin to be a stably dispersed in a liquid medium over a long term. And further, in order to satisfy these requirements, it is necessary for the aqueous pigment dispersions to possess excellent dispersing stability, which enables the aqueous pigment dispersions to be stored for a long term.
Recently, therefore, in these days, various investigations of aqueous pigment dispersions as to a composition suitable for ink-jet recording or a dispersing method of a pigment have started.
The applicant of the present application proposed a styrene-acrylic type resin having an acid value ranging from 50 to 280, as a resin composition suitable for forming colored particles by microcapsulating a pigment with a phase inversion emulsifying method (for example, see Japanese Unexamined Patent Application, First Publication No. H08-183920 (claims)), and disclosed that a resin which has a monomer constitution including a styrene monomer of 60 to 90 moles, an acrylic monomer of 5 to 15 moles, and a methacrylic monomer of 5 to 25 moles excels in dispersing stability and jetting performance. However, the production method using such a resin through a phase inversion emulsifying method is not suitable for performing mass-production stably, because it necessitates a step of evaporating the solvent used for dissolving the resin, which complicates the producing process. Moreover, in the case in which the solvent which cannot evaporate off remains in the aqueous ink for ink-jet recording, it causes a solvent-smelling or aggregation of dispersing particles in the aqueous ink for ink-jet recording, and accordingly a process for producing aqueous pigment dispersions which excel in mass-production efficiency and dispersing stability has been desired.
As such a production method, for example, a method which includes preparing an aqueous solution in which a water-soluble resin and an alkali component are dissolved, adding pigments thereto and stirring it sufficiently, and thereafter dispersing it using a high-speed sand mill with higher dispersing efficiency, etc., to obtain aqueous pigment dispersions is proposed (for example, see Japanese Unexamined Patent Application, First Publication No. 2001-262038(page 234)).
However, the step of producing aqueous pigment dispersions using a sand mill is generally performed using a dispersed liquid of low viscosity having a small percentage of solid content such as a pigment, etc. Accordingly, a strong sharing force is hardly applied to pigments, thereby it takes a long time to crush large pigment particles.
Moreover, aqueous pigment dispersions thus obtained contain a considerable amount of large particles having a particle size of not less than 1 μm even after being dispersed. And since the jetting stability of the ink-jet cannot be secured in this state, it is necessary to perform a further step of removing these coarse particles using centrifugation, filtration, etc., and as a result, there is a problem in that production efficiency and yield further decrease.
In addition, the applicant produced aqueous pigment dispersions in which carbon black was dispersed, using a resin having a weight average molecular weight of 7200 with the resin composition disclosed in the above patent official report 1, by a method using a paint shaker, which uses beads similarly to a sand mill (for example, see Japanese Unexamined Patent Application, First Publication No. 2002-256201 (claims, Examples)). According to this production method for aqueous pigment dispersions, it is possible to produce an aqueous ink for ink-jet recording which has a fine particle size and excellent dispersing stability, and the ink jetting performance is also excellent. However, in the case in which this production method is applied to azo pigments or quinacridone pigments which are generally difficult to be dispersed, the dispersing performance is not as good as that of carbon black. Moreover, since this method is only suitable for small scale production, it is not possible to perform mass-production of the aqueous pigment dispersions efficiently.
On the other hand, a method is known which includes kneading a mixture of a resin and pigment or a mixture of an aqueous resin solution which contains a resin, water, and a water-soluble organic solvent and pigment through rolls, before the dispersing step is performed. In a twin-roll, a process is performed which includes kneading the above mixture to produce solid chips, then adding mainly water and a water-soluble organic solvent to the resultant chips, and then dispersing the mixture using a high-speed mixer or a homogenizer, etc., to obtain aqueous pigment dispersions (for example, see Japanese Unexamined Patent Application, First Publication No. H6-157954 (pages 2, 3, 5 and 6) and Japanese Unexamined Patent Application, First Publication No. 2000-80299 (pages 2 and 3)).
In addition, adding an organic amine in order to make production of a resin solution easy is also performed (for example, see Japanese Unexamined Patent Application, First Publication No. 2001-81390 (page 5)).
For example, aqueous pigment dispersions are produced through a kneading process by a twin-roll, using a styrene-acrylic type resin having a weight average molecular weight of 50000 and the resin composition disclosed in Japanese Unexamined Patent Application, First Publication No. H10-88042.
If such a method is used, then pigments are subjected to a shearing force between the rolls to be finely crushed, however, since open type kneading is actually performed, water and water-soluble organic solvent evaporate in the kneading step, thereby finally forming solid chips having a high solid content percentage. And as a result, in the subsequent, it is necessary to perform crushing and dissolving of the solid chips and dispersing of pigments by adding water and a water-soluble organic solvent.
Accordingly, this burdens the dispersing step subsequent to the step of kneading with rolls, and as a result, there is possibility that dispersing time may be prolonged, or, even if dispersing is performed for a long time, large particles may remain. Moreover, the solid chips after being kneaded between the rolls is processed to the dispersing step in which the solid chips is crushed and dissolved in this way, and hence even if the surface of the pigment is coated with resin, the resin coating on the surface of pigment after production of the aqueous pigment dispersions may not necessarily be sufficient.
Moreover, in the kneading step using the twin-rolls, the kneaded material should be shaped into a sheet between the rolls in the kneading step and the kneaded material should not be eliminated from the rolls. For this reason, there is a possibility that the raw material, pigment, resin, water, water-soluble organic solvent, etc., and blending percentage thereof may be restricted, because there is a problem in that the colored kneaded product may not successfully become uniform, depending on the thermal characteristic of the resin, and on blending percentage of the raw material.